1. Field of the Invention
The present invention relates to a battery voltage monitoring apparatus that detects a battery voltage of a power supply device including a plurality of secondary batteries connected in series. In particular, the present invention relates to a battery voltage monitoring apparatus that detects an abnormal voltage related to voltage measurement.
2. Description of Related Art
Conventionally, an electric vehicle and a hybrid vehicle have been known as environmentally friendly vehicles. A motor is used as a drive source for driving such an electric vehicle and hybrid vehicle. The motor is connected with a rechargeable secondary battery. A DC voltage obtained from the secondary battery is converted into an AC voltage, whereby the motor is driven. Since the secondary battery requires a high voltage, the secondary battery is generally implemented as a battery pack having a plurality of battery cells connected in series.
In order to detect voltages of the battery cells of the battery pack, a plurality of voltage sensors are used. The voltage sensors are divided into an appropriate number of groups to be modularized. When a large number of battery cells to be connected in series are required for electric vehicles and the like, a plurality of voltage sensor modules are also prepared and connected in series. Japanese Unexamined Patent Application Publications Nos. 09-159701 and 2003-092840 each disclose an apparatus for monitoring a voltage of such a battery pack. A description is given below of the apparatus for monitoring the voltage of the battery pack, in which the module including the plurality of voltage sensors is formed of a single semiconductor device (IC) and a plurality of semiconductor devices are connected to each other.
FIG. 8 shows a voltage monitoring apparatus for monitoring an output voltage of a battery pack according to the related art. The battery pack includes a plurality of battery cells connected in series, and a plurality of battery terminals. Outputs of the battery cells are connected to corresponding battery terminals. FIG. 8 also shows an example where 80 battery cells each having an electromotive voltage of 3.6 V are connected in series.
The voltage monitoring apparatus includes voltage sensor modules (ICs), input terminals, and output terminals. The input terminals are connected to the battery terminals of the battery pack and to module input terminals of the voltage sensor modules so as to receive voltages output by the battery cells and supply the voltages to each of the voltage sensor modules. The output terminals are connected to module output terminals of the voltage sensor modules so as to output monitoring results of the voltage sensor modules to an outside of the voltage monitoring apparatus. In FIG. 8, the voltage monitoring apparatus includes 20 voltage sensor modules each capable of monitoring output voltages of four battery cells, thereby making it possible to monitor output voltages of all the 80 battery cells included in the battery pack.
FIG. 9 shows a voltage sensor module for monitoring output voltages of four battery cells according to the related art. The voltage sensor module includes two types of comparators (overvoltage detection comparator and undervoltage detection comparator) and reference voltage generation circuits. In the voltage sensor module, a reference voltage output from each of the reference voltage generation circuits is compared with a voltage generated based on an output voltage of each battery cell, whereby undervoltage detection and overvoltage detection are performed with respect to an output voltage of each battery cell. The undervoltage detection and the overvoltage detection for a single battery cell require each one of the reference voltage generation circuit, the undervoltage detection comparator, and the overvoltage detection comparator.
Each reference voltage generation circuit is connected between the module input terminals connected to both ends (positive electrode side and negative electrode side) of a single battery cell via the battery terminals of the battery pack and the input terminals of the voltage monitoring apparatus so as to generate a reference voltage based on a voltage applied between the module input terminals. Between the module input terminals, in parallel with the reference voltage generation circuit, three resistors are connected in series, and two types of resistance-divided voltages are obtained by resistance voltage division.
In this case, by focusing on a battery cell A of FIG. 9, a detailed description is given of a connection relation between the reference voltage generation circuit (Vref), the undervoltage detection comparator, and the overvoltage detection comparator, and operations thereof (undervoltage detection and overvoltage detection).
A positive electrode side of the battery cell A is electrically connected to a module input terminal N3 of the voltage sensor module via an external terminal N1 of the battery pack and an input terminal N2 of the voltage monitoring apparatus. On the other hand, a negative electrode side of the battery cell A is electrically connected to a module input terminal N6 of the voltage sensor module via an external terminal N4 of the battery pack and an input terminal N5 of the voltage monitoring apparatus. Moreover, between the module input terminals N3 and N6, three resistors connected in series are connected in parallel with the reference voltage generation circuit VrefA. Note that, as shown in FIG. 9, when it is assumed that nodes between the resistors connected in series correspond to points A and B, a voltage applied between the module input terminals N3 and N6 is divided by the resistors (resistance-divided voltage at point A is larger than resistance-divided voltage at point B).
An inverting input terminal of an undervoltage detection comparator A is connected to the point A, and a non-inverting input terminal of the undervoltage detection comparator A is connected to the VrefA. On the other hand, an inverting input terminal of an overvoltage detection comparator A is connected to the VrefA, and a non-inverting input terminal of the overvoltage detection comparator A is connected to the point B. In addition, outputs of the undervoltage detection comparator A and the overvoltage detection comparator A are connected to module output terminals N7 and N8 of the voltage sensor module, respectively.
A reference voltage and resistance output by the VrefA are set to such values that the undervoltage detection comparator A and the overvoltage detection comparator A each output an “L” level signal indicating a normal state, when an output voltage of the battery cell A falls within an allowable voltage range. Accordingly, when the output voltage of the battery cell A decreases to an undervoltage side from the allowable range (when output voltage decreases to an abnormal value), the resistance-divided voltage at the point A also decreases to an abnormal value. The undervoltage detection comparator A compares the resistance-divided voltage at the point A, which is an abnormally low voltage, with the reference voltage, and outputs an “H” level signal indicating that the output voltage abnormally decreases to the undervoltage side. On the contrary, when the output voltage of the battery cell A increases to an overvoltage side from the allowable range (when output voltage increases to an abnormal value), the resistance-divided voltage at the point B also increases to an abnormal value. The overvoltage detection comparator A compares the resistance-divided voltage at the point B, which is an abnormally high voltage, with the reference voltage, and outputs an “H” level signal indicating that the output voltage increases abnormally to the overvoltage side. In this manner, the voltage monitoring apparatus of the related art detects an abnormality when the output voltage of the battery cell becomes undervoltage or overvoltage.
Note that Japanese Unexamined Patent Application Publication Nos. 09-159701 and 2003-092840 disclose an overvoltage detecting apparatus for a battery pack and a battery pack controller, respectively.
However, when the output voltage of the battery cell decreases to or below a predetermined level, the reference voltage generation circuit for generating the reference voltage based on the battery cell that causes a voltage drop becomes incapable of generating an appropriate reference voltage. In a case where the appropriate reference voltage is not generated, normal operations of an undervoltage detection system using the undervoltage detection comparator cannot be expected.
FIG. 10 shows a potential at the point A, an output of the VrefA, and an output of the undervoltage detection comparator A, with respect to a variation in output voltage of the battery cell A. When the output voltage of the battery cell A is equal to larger than V1, the potential at the point A becomes larger than the output voltage of the VrefA. In this case, the undervoltage detection comparator A produces an “L” level output, and it is detected that the voltage within a normal range is output by the battery cell A (undervoltage is not detected).
When the output voltage of the battery cell A is equal to or lower than V1, the output voltage of the battery cell A becomes an abnormal voltage to the undervoltage side. When the output voltage of the battery cell A becomes V2, it is impossible to generate a threshold voltage (reference voltage) for detecting the undervoltage based on the reference voltage. As a result, the output voltage of the battery cell A decreases, and at the same time, the output voltage of the VrefA gradually decreases. During a time when the output voltage of the VrefA is higher than the potential at the point A (between V3 and V1), the undervoltage detection comparator produces an “H” level output. Accordingly, it is possible to normally detect that the output voltage of the battery cell A is undervoltage. However, when the output voltage of the battery cell A is equal to or lower than V3, the output voltage of the VrefA further decreases below the potential at the point A. In this case, even though the output voltage of the battery cell A is an abnormal voltage, the undervoltage comparator A produces the “L” level output, and it is detected that the battery cell A outputs the voltage within the normal range.
Thus, in the undervoltage detection system of the voltage monitoring apparatus for a battery pack according to the related art, there is a problem in that, when an output voltage of a battery cell serving as a detection target decreases to or below the predetermined level, it is impossible to detect an undervoltage abnormality of the battery cell.